Visual abilities decline during aging. Although some of these declines are due to optical changes, many are neural in origin. A common finding in studies of aging is that there are wide individual differences. Some old individuals are indistinguishable from young adults, whereas others show clear age-related changes. One broad goal of the proposed research is to understand the nature of the neural changes in the visual system during aging, how they lead to declines in visual abilities, and how neural function differs between individuals with visual disabilities and those whose vision is unchanged during aging. A second goal is to use the visual system as a model to study the relationship between changes in brain and behavior during aging. Rhesus monkeys will be used for these studies because their visual system is very similar to that of humans, and the use of an animal model will allow us to carry out combined behavioral, physiological, and anatomical studies that are not possible in humans. The proposed experiments have three specific scientific aims: 1) Use noninvasive sweep visual evoked-potential (sweep VEP) methods to evaluate visual function in a large number of young adult and old rhesus monkeys (approximately 50 each). The sweep-VEP testing will provide information about how various measures of visual function in the retina and central visual pathways are affected by aging in a large sample of rhesus monkeys. In addition, we will use the sweep-VEP estimates of visual function to select three groups of five to six monkeys for further studies of behavior, neurophysiology, and neuroanatomy. The three groups are average young adults, old monkeys that show clear deficits in visual function measured with the sweep VEP, and old monkeys that show no deficits in the sweep-VEP measures of visual function. 2) Carry out extensive behavioral psychophysical studies of the visual abilities of these three groups of monkeys. Performance will be compared to determine the effects of aging, individual differences in these effects, and how performance on the behavior tests relates to the sweep-VEP measures. 3) Carry out neurophysiological and neuroanatomical studies of the animals studied behaviorally. In addition, if the sweep VEP is a good predictor of behavioral abilities (as expected from previous studies of humans), we will use it to screen animals for anatomical and physiological studies without prior behavioral testing. These studies will allow us to relate neuronal structure, neurophysiological processing, and behavior in the same animals and to begin testing hypotheses, based on previous behavioral studies in humans, concerning the location and nature of neural deficits during aging. They also will provide a basis for future investigations of how the visual system is affected by a variety of age-related diseases, such as glaucoma, Alzheimer's disease, and Parkinson's disease.